1. Field of the Invention
This invention generally relates to methods for the synthesis of superfine materials having particle or gain dimensions larger than 100 nm and less than or equal to about 10 microns. In particular, this invention relates to chemical methods for the introduction of a grain growth inhibitor and/or alloy additions into superfine materials, resulting in materials having controlled microstructure and morphology. The obtained materials exhibit superior properties, including improved fracture toughness, hardness, and wear-, corrosion-, and erosion-resistance.
2. Brief Description of the Prior Art
For decades, materials with fine-scale microstructures have been recognized to exhibit unusual and technologically attractive properties. Currently, interest is growing in classes of materials composed of very fine grains or particles having dimensions in the range of less than or equal to 100 nanometers (nm), known as xe2x80x9cnanostructuredxe2x80x9d materials; materials composed of grains or particles having dimension in the range of greater than 100 nm and less than or equal to 1 micron, known as xe2x80x9cultrafinexe2x80x9d materials; and a class of materials having dimensions between about 1 micron and about 10 microns. As used hereinafter, xe2x80x9csuperfinexe2x80x9d materials will refer collectively to materials having dimensions of greater than 100 nm and less than or equal to about 10 microns. A feature of superfine materials is the high fraction of atoms that reside at grain or particle boundaries compared to larger materials. Nanostructured and superfine materials may thus have substantially different, often superior chemical and physical properties compared to conventional, large-sized counterparts having the same composition. Thus considerable advantages accrue from the substitution of nanostructured and superfine materials for conventional large-structured materials in a wide range of applications, for example superior strength, improved fracture toughness and hardness in martensitic steels, and reduced sintering temperature for consolidation and the onset of superplasticity in ceramics. Nanostructured and superfine metal alloys and metal carbides, in particular, are expected to have superior properties, including improved fracture toughness, hardness, and wear-, corrosion-, and erosion-resistance. The ability to synthesize and optimize the pore structure and packing density of nanostructured and superfine materials at the atomic level is a powerful tool for obtaining new classes of these materials. These new classes, together with designed multifunctional coatings, present unprecedented opportunities for advances in material properties and performance for a broad range of engineering applications.
Inframat Corp. has made significant progress in the field of nanostructured and superfine materials, including in the synthesis of nanostructured metal powders by the organic solution reaction (OSR) method, aqueous solution reaction (ASR) method, and in advanced chemical processing of oxides and hydroxides materials for structural, battery and fuel cell applications. Examples of materials produced from these methods include nanostructured alloys of Ni/Cr, nanostructured NiCr/Cr3C2 composites, nanostructured yttria-stabilized ZrO2, nanofibrous MnO2, and Ni(OH)2. Inframat has further developed technologies for manufacturing nanostructured and ultrafine materials in bulk quantities as disclosed in xe2x80x9cNanostructured Oxide and Hydroxide Materials and Methods of Synthesis Therefor,xe2x80x9d which is the subject of pending U.S. and foreign applications (including U.S. Ser. No. 08/971, filed Nov. 17, 1997), as well as technologies for the thermal spray of nanostructured and ultrafine feeds including nanostructured WCxe2x80x94Co composites, as disclosed in xe2x80x9cNanostructured Feeds for Thermal Spray Systems, Method of Manufacture, and Coating Formed Therefromxe2x80x9d also the subject of pending U.S. and foreign patent applications (including U.S. Ser. No. 09/019,061, filed Feb. 5, 1998), both U.S. patent applications being incorporated herein in their entirety. Chemical syntheses of nanostructured metals, ceramics, and composites using OSR and ASR methods have also been previously described by Xiao and Strutt in xe2x80x9cNanostructured Metals, Metal Alloys, Metal Carbides and Metal Alloy Carbides and Chemical Synthesis Thereof,xe2x80x9d U.S. Ser. No. 08/989,000, filed Dec. 5, 1996, incorporated herein by reference, as well as in xe2x80x9cSynthesis and Processing of Nanostructured Ni/Cr and Nixe2x80x94Cr3C2 Via an Organic Solution Method,xe2x80x9d Nanostructured Mater. Vol. 7 (1996) pp. 857-871 and in xe2x80x9cSynthesis of Si(C,N) Nanostructured Powders From an Organometallic Aerosol Using a Hot-Wax Reactor,xe2x80x9d J. Mater. Sci. Vol. 28 (1993), pp. 1334-1340.
The OSR and ASR methods employ a step-wise process generally comprising (1) preparation of an organic (OSR) or aqueous (ASR) solution of mixed metal halides; (2) reaction of the reactants via spray atomization to produce a nanostructured precipitate; and (3) washing and filtering of the precipitate. The precipitate is often then heat treated, and/or subjected to gas phase carburization under either controlled carbon/oxygen activity conditions (to form the desired dispersion of carbide phases in a metallic matrix phase), or under controlled nitrogen/hydrogen activity conditions to form nanostructured nitrides. This procedure has been used to synthesize various nanostructured compositions, including nanostructured NiCr/Cr3C2 powders for use in thermal spraying of corrosion resistant hard coatings. An advanced chemical processing method combines the ASR and OSR methods with spray atomization and ultrasonic agitation.
Another approach to the synthesis of nanostructured materials is the inert gas condensation (IGC) method. As described in xe2x80x9cMaterials with Ultrafine Microstructures: Retrospective and Perspectivesxe2x80x9d, Nanostructured Materials Vol. 1, pp. 1-19, Gleiter originally used this method to produce nanostructured metal and ceramics clusters. The method was later extensively used by Siegel to produce nanostructured TiO2 and other systems, as described in xe2x80x9cCreating Nanophase Materialsxe2x80x9d, Scientific American Vol. 275 (1996), pp.74-79. This method is the most versatile process in use today for synthesizing experimental quantities of nanostructured metals and ceramic powders. The IGC method uses an evaporative sources of metals, which are then convectively transported and collected on a cold substrate. Ceramic particles must be obtained by initially vaporizing the metal source, followed by a slow oxidation process. A feature of this method is the ability to generate loosely agglomerated nanostructured powders, which are sinterable at low temperatures.
One other method for the synthesis of nanostructured materials is chemical vapor condensation (CVC). CVC is described by Kear et al. in xe2x80x9cChemical Vapor Synthesis of Nanostructured Ceramicsxe2x80x9d in Molecularly Designed Ultra fine/Nanostructured Materials in MRS Symp. Proc. Vol. 351 (1994), pp. 363-368. In CVC, the reaction vessel is similar to that used the IGC method, but instead of using an evaporative source, a hot-wall tubular reactor is used to decompose a precursor/carrier gas to form a continuous stream of clusters of nanoparticles exiting the reactor tube. These clusters are then rapidly expanded out to the main reaction chamber, and collected on a liquid nitrogen cooled substrate. The CVC method has been used primarily with chemical precursors or commercially available precursors. Kear describes the production of nanostructured SiCxNy and oxides from hexamethyldisilazane.
Finally, a thermochemical conversion method for producing nanostructured WCxe2x80x94Co has been disclosed by Kear in xe2x80x9cSynthesis and Processing of Nanophase WCxe2x80x94Co Compositexe2x80x9d Mater. Sci. Techn. Vol. 6 (1990), p. 953. In this method, aqueous solutions containing tungsten and cobalt precursors are spray-dried to form an intermediate precursor at temperatures of about 150 to 300xc2x0 C. This intermediate precursor is a mixture of amorphous tungsten oxide and cobalt in the form of a spherical hollow shell having a diameter of about 50 microns and a wall thickness of about 10 microns. Nanostructured WCxe2x80x94Co is then obtained by the carburization of this precursor powder at 800-900xc2x0 C. in a carbon monoxide/carbon dioxide mixture. The synthesis of nanostructured WC/Co using this technique has described in several patents by McCandlish et al., including xe2x80x9cMultiphase Composite Particle,xe2x80x9d U.S. Pat. No. 4,851,041, and xe2x80x9cCarbothermic Reaction Process for Making Nanophase WCxe2x80x94Co,xe2x80x9d U.S. Pat. No. 5,230,729. Synthesis of nanostructured and superfine WC/Co is of particular interest to industry, as is it is presently the material of choice for cutting tool, drill bit, and wear applications.
It is expected that a number of the techniques developed especially for nanostructured materials are also applicable to synthesis of superfine materials, through controlled manipulation of grain size either at the synthetic level or through the use of grain growth inhibitors. However, a major drawback of the above-described techniques, as well as for other techniques for the synthesis of superfine materials known in the art, is the tendency of the produced materials to undergo uncontrolled grain growth during sintering or use of the nanostructured component, especially at high temperature. For example, the tungsten carbide grains of as-synthesized nanostructured WCxe2x80x94Co have diameters of about 30 nm. During liquid phase sintering, the tungsten carbide grains grow rapidly to diameters of several microns or larger in a relatively short time, e.g., a few minutes. After exhaustive research and engineering evaluations it has been concluded that VC and/or Cr3C2 are the most effective carbide phases of the very large range of materials evaluated over the years.
Vanadium carbide, for example, has been employed by Nanodyne, Inc. (Brunswick, N.J.) to prevent this disadvantageous grain growth, as described in Nanodyne""s product specification for the product sold under the tradename Nanocarb(copyright). In this process, micron-sized vanadium carbide powders are blended into the WC/Co powder composite via mechanical mixing, followed by shape-formation and sintering into bulk components. The use of vanadium carbide is effective to prevent some degree of grain growth, as the final grain size of the consolidated bulk piece is in the sub-micron range, e.g., 0.2-0.8 microns. The major drawback of this grain growth technique is the non-homogeneous mixing of the VC within the WC/Co composite materials as well as the difficulty of sintering kinetics, resulting in non-homogeneous bulk material properties, or increased cost for sintering procedure.
In an attempt to solve the above problem, Nanodyne currently employs a chemical precipitation technique in which a vanadium salt is introduced chemically at the start of the materials synthesis. Although use of this technique overcomes the problem of non-homogeneous mixing, it produces vanadium oxide instead of vanadium carbide. It is well known that any oxide material that presented in the WC/Co system is detrimental to the material. The introduction of vanadium oxide into the WC/Co system not only creates sintering difficulties, but also requires an extra carbon source in the WC/Co powder for the conversion of vanadium oxide into vanadium carbide at extremely high temperatures, e.g., 1450xc2x0 C. In many cases the extra carbon source embrittles the consolidated materials.
Prior incorporation of boron compounds into fine-grained materials has been described in xe2x80x9cSynthesis of AlN/BN Composite Materialsxe2x80x9d by Xiao and Strutt, in J. Am. Ceram. Soc., Vol. 76, p. 987 (1993), which discloses synthesis from a composite precursor comprising aluminum, boron, and nitrogen. The boron nitride polymer is formed by bubbling ammonia into an aqueous solution of boric acid and urea.
Scoville et al., in xe2x80x9cMolecularly Designed Ultrafine/Nanostructured Materialsxe2x80x9d, ed. by K. E. Gonsalves, D. M. Chow, T. D. Xiao, and R. Cammarato, MRS Symp. Vol. 351, p. 431 (1994) describe a method for incorporation of BN nanostructured particles into a germanium crystal lattice to significantly reduce thermal conductivity. In this method, micron-sized BN and Si/Ge powders are blended together and evaporated using a plasma torch to form mixtures, which are then condensed into a composite of BN/Si/Ge. Heat treatment results in large crystals ( greater than 1 micron) of Si/Ge wherein discrete 2-10 nm BN grains are trapped inside the large crystals.
Boron nitride has also been incorporated into a conventional (larger than 100 nm grain size) titanium diboride system with WC/Co additives as disclosed in U.S. Pat. No. 5,632,941 to Mehrotra et al. BN is incorporated in the form of a powder. U.S. Pat. No. 4,713,123 further disclose use of BN as a grain growth inhibitor in conventional (larger than 100 nm grain size) silicon steel. However, when the quantity of BN is too large, it is becomes difficult to grow the secondary recrystallized grain with {110} less than 001 greater than orientation, so that the amount is limited to a range of 0.0003-0.02%. Addition of boron to silicon steel in the form of ferroboron, followed by nitridation of the steel to provide nitrogen results in the formation of slight amounts of boron or BN, which may inhibit grain boundary migration, as reported by Grenoble in IEEE Trans. Mag., May 13th, p. 1427 (1977), and Fiedler in IEEE Trans. Mag. May 13th p. 1433 (1977). None of these references disclose use of a BN polymer as a grain growth inhibitor precursor.
Accordingly, there remains a need in the art for methods of inhibiting and/or controlling grain growth during the processing of as-synthesized nanostructured and superfine materials and nanostructured and superfine material intermediates, especially a method applicable to a wide range of compositions.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by the method of the present invention, comprising incorporation of a polymer precursor of a grain growth inhibitor into superfine materials or intermediates useful for the production of superfine materials. The precursor/nanostructured (or superfine) material composite is optionally heat treated at a temperature below the grain growth temperature of the material in order to more effectively disperse the precursor. The composites are then heat treated at a temperature effective to decompose the precursor, thereby forming materials having grain growth inhibitors uniformly distributed therein, preferably onto the grain boundaries. Synthesis of a preferred inorganic polymer solution comprises forming an inorganic polymer from a solution of metal salts, filtering the polymer, and drying.
In a first embodiment of the present invention, the inorganic polymer precursor is introduced into the superfine material in the powder production step, i.e., during the synthesis of the material or an intermediate leading to the product material, and then converted by heat treatment to the grain growth inhibition species, thus allowing the grain growth species to be homogeneously mixed with the material at the molecular level. Such heat treatment may also be used to convert an intermediate superfine material into the product superfine material.
In a second embodiment, the polymeric precursor is incorporated into an already-synthesized superfine material or intermediate. In one method, the polymer precursor is dissolved into a solvent containing a dispersion of already-synthesized particles and the resultant slurry is oven-dried or spray-dried to form a dried powder having the grain growth inhibitor uniformly distributed within the grain boundaries of the superfine particles. Alternatively, already-synthesized particles are coated with the polymer precursor and optionally heat treated at a temperature lower than the grain growth temperature, thereby melting and diffusing precursor through any matrix and to the grain boundaries. The oven-dried, spray-dried or coated particles/precursor composites are then heat treated if necessary to convert the polymer precursor to grain growth inhibitor, and optionally further processed (e.g., by nitridation or carburization) to produce product materials having grain growth inhibitors uniformly distributed at the grain boundaries. This methodology is capable of substantially coating each particle with a grain growth inhibitor barrier, or of obtaining homogeneous mixing of the grain growth inhibitor with the nanoparticles at the nanometer level.
In still another embodiment of the invention, alloying additives and/or grain growth inhibitors are incorporated into the materials. The alloying additives and/or grain growth inhibitor precursor may be incorporated into the reaction mixture used to synthesize the material as described in the above first embodiment, or the alloying additives precursor may be incorporated into the as-synthesized material as described in the above second embodiment. The as-synthesized material may comprise grain growth inhibitor or precursor incorporated at the synthesis stage. Alternatively, the alloying additive is combined with the polymeric grain growth inhibitor precursor and then incorporated into the already-synthesized particles. Mixing, e.g., by ball-milling to form a homogeneous mixture of the particles with the alloying additives and the grain growth inhibitor precursor is followed by spray drying or oven-drying and processing as described above for the second embodiment.
An especially advantageous feature of the present method is its applicability to a wide variety of materials systems, including metals, metal alloys, carbides, nitrides, intermetallics, ceramics, and their combinations. Preferably, the grain growth inhibitor itself is a high performance material, exhibiting excellent mechanical and other physical and chemical properties. Addition of alloying additives further improves the properties of the product materials, including hardness, toughness, density, corrosion- and erosion-resistance. The present invention allows the economical, large-scale fabrication of high performance superfine materials having a wide range of compositions for targeted applications.